From: Andrew Morton <akpm@linux-foundation.org>
To: David Rientjes <rientjes@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Davidlohr Bueso <dave@stgolabs.net>,
Alexey Dobriyan <adobriyan@gmail.com>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [patch] mm, vmacache: hash addresses based on pmd
Date: Mon, 9 Jul 2018 18:08:41 -0700 [thread overview]
Message-ID: <20180709180841.ebfb6cf70bd8dc08b269c0d9@linux-foundation.org> (raw)
In-Reply-To: <alpine.DEB.2.21.1807091749150.114630@chino.kir.corp.google.com>
On Mon, 9 Jul 2018 17:50:03 -0700 (PDT) David Rientjes <rientjes@google.com> wrote:
> When perf profiling a wide variety of different workloads, it was found
> that vmacache_find() had higher than expected cost: up to 0.08% of cpu
> utilization in some cases. This was found to rival other core VM
> functions such as alloc_pages_vma() with thp enabled and default
> mempolicy, and the conditionals in __get_vma_policy().
>
> VMACACHE_HASH() determines which of the four per-task_struct slots a vma
> is cached for a particular address. This currently depends on the pfn,
> so pfn 5212 occupies a different vmacache slot than its neighboring
> pfn 5213.
>
> vmacache_find() iterates through all four of current's vmacache slots
> when looking up an address. Hashing based on pfn, an address has
> ~1/VMACACHE_SIZE chance of being cached in the first vmacache slot, or
> about 25%, *if* the vma is cached.
>
> This patch hashes an address by its pmd instead of pte to optimize for
> workloads with good spatial locality. This results in a higher
> probability of vmas being cached in the first slot that is checked:
> normally ~70% on the same workloads instead of 25%.
Was the improvement quantifiable?
Surprised. That little array will all be in CPU cache and that loop
should execute pretty quickly? If it's *that* sensitive then let's zap
the no-longer-needed WARN_ON. And we could hide all the event counting
behind some developer-only ifdef.
Did you consider LRU-sorting the array instead?
next prev parent reply other threads:[~2018-07-10 1:08 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-07-10 0:50 David Rientjes
2018-07-10 1:08 ` Andrew Morton [this message]
2018-07-10 1:37 ` David Rientjes
2018-07-11 23:10 ` Andrew Morton
2018-07-11 23:43 ` David Rientjes
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20180709180841.ebfb6cf70bd8dc08b269c0d9@linux-foundation.org \
--to=akpm@linux-foundation.org \
--cc=adobriyan@gmail.com \
--cc=dave@stgolabs.net \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=rientjes@google.com \
--cc=torvalds@linux-foundation.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox